Process for producing silver nitrate



Dec. 7, 1965 v. SOORYN PROCESS FOR PRODUCING SILVER NITRATE Filed March 21, 1960 mkbOwCbmJm Vsevolod Sooryn IN VEN TOR.

AGENT.

United States Patent 3,222,128 PROCESS FOR PRODUCING SILVER NITRATE Vsevolod Sooryn, Paris, France, assignor to Benjamin B. Doell, New York, N.Y. Filed Mar. 21, 1960, Ser. No. 16,304 6 Claims. (Cl. 23-102) My present invention relates to a process for producing silver nitrate as more generally disclosed in my co-pending application Ser. No. 779,521, filed Dec. 11, 1958, of which the present application is a continuation-in-part, now abandoned.

The aforementioned co-pending application discloses a process wherein crystals of high-purity silver nitrate are produced from metallic silver through the use of a silveroxide intermediary. The latter is deposited at an inert electrode in an alkaline electrolyte and, after its removal therefrom, contacted with nitric acid to be converted to silver nitrate.

It is an object of the present invention to provide an efficient process for the large-scale production of highpurity nitric acid in the manner described above.

In accordance with a feature of the invention, silver oxide is electrolytically deposited upon a continuous inert but flexible band which passes from the alkaline electrolyte through a water bath to remove traces of alkalinity and thence into a bath of nitric acid in which the silver oxide is stripped from the band and crystals of silver nitrate are precipitated. The band then passes into another water bath wherein excess nitric acid is washed from the band which is thereupon returned to the electrolyte.

According to another feature of the invention, the bath of preferably concentrated nitric acid (48.5 B) is maintained at a temperature in excess of 100 C. to drive oflf the water resulting from the reaction between the nitric acid and the silver oxide. Only a small proportion of the length of the continuous band is permitted to enter the nitric-acid bath whose volume is kept at a minimum in order to facilitate recovery of the silver-nitrate crystals from the liquor and to limit the erosive action of the acid upon the band.

I have found that the silver-oxide mass electrodeposited upon the cathodized flexible band tends to adhere thereto as it passes out of the electrolyte, especially if the mass is compressed against the band as, for example, by a pair of rollers. The latter may also serve to free the oxide mass from electrolyte via a squeegee action. The silver-oxide mass may, therefore, be carried along an extended transport path by the band.

The above and other objects, features and advantages of the present invention will become more fully apparent from the following description, reference being made to the accompanying drawing in which the sole figure is a schematic, cross-sectional view of a plant for producing silver nitrate by a process according to the invention.

In the figure I show a processing tank 10 divided by partition walls 11, 12 and 13 into a first compartment 14 containing an alkaline electrolyte, a second compartment 15, adjacent thereto, containing a water bath, a third compartment 16 containing a nitric-acid bath, and a fourth compartment 17 containing another water bath. An inert, flexible and conductive band 18, preferably formed from a stainless steel resistant to attack by both the alkaline electrolyte and the nitric-acid bath, passes over guide rollers 19 and between a pair of drive rollers 20, bearing thereupon, into the electrolyte 21 of compartment 14. The electrolyte 21 is, preferably, an aqueous solution of an alkali, such as potassium hydroxide, in which a salt having an ion capable of forming soluble silver compounds is dissolved. An example of such salt is potassium (or sodium) acetate which may be readily dissolved 3,222,128 Patented Dec. 7, 1955 in the electrolyte and which may be present in a proportion up to the point of saturation. Advantageously, the concentration of alkali in the solution is maintained at the value corresponding to maximum conductivity of the electrolyte. Therefore, a 27%-to-30% solution of potassium hydroxide, having a specific gravity between 1.270 and 1.305 at room temperature, will be found to be satisfactory.

A pair of silver electrodes 22 (only one of which is shown) are suspended at opposite sides of the band 18 which is brought into an upright position in the compartment 14, as it passes between a pair of vertical rollers 23,- so that electrodeposition of silver oxide may occur on both surfaces of the band. A second pair of vertical rollers 24 then serve to compress the silver-oxide mass against the band to limit the flaking off of the mass as the band 18 leaves the compartment 14. As the band leaves this compartment, it passes between a pair of horizontal rollers 25 which further compress the mass and squeeze adherent electrolyte therefrom.

From the compartment 14, the band is guided over an idler roller 26 and into the water bath 27 contained within the compartment 15. The band 18 is led under two rollers 28 immersed in the water bath 27 and, upon emerging therefrom, between another pair of squeegee rollers 29, similar to rollers 25, which serve to extract the water from the mass carried by the band. The compartment 15 is provided with a water inlet 30 and an outlet 31 to permit the water bath 27 to be changed continuously or intermittently as required.

The band 18, carrying the silver-oxide mass which has been washed substantially free from all traces of the alkaline electrolyte in the water bath 27, is then directed over an idler roller 32 between compartments 15 and 16 and down into a nitric-acid bath 33 and around a roller 34 immersed therein. Since the reaction between the silver-oxide mass and the nitric acid is rapid, it is not necessary to provide more than a rather shallow bath. The temperature of the bath 33 is maintained, with the aid of suitable heating means (not shown) and a thermostat 50, at or slightly above C. to evaporate, as rapidly as it is formed the water resulting from the reaction occurring therein. Additional nitric acid may be added through inlet 35, while the silver-nitrate crystals may be drawn off in the liquor via outlet 36. Upon leaving the nitric-acid bath 33, band 18 travels between a pair of scraper blades 37, bearing thereupon, which strip adherent silver-nitrate crystals from the band and also wipe off nitric-acid droplets.

The band 18 is washed free of acid in the water bath 40 of compartment 17 into which it passes over an idler roller 38. Two rollers 39 immersed in the bath 40 entrain the band 18 beneath the wash liquid. This liquid may be changed through an inlet 42 and an outlet 43 provided in the wall of compartment 17. From the bath 40, the band passes between two strippers 41 which remove water droplets adhering thereto to prevent dilution of the electrolyte 21 to which the band is guided over rollers 19.

The silver electrodes 22 are connected to a directcurrent source 45 to form the anodes, while the band 18 is contacted by a wiper 44 connected to the negative pole of the source 45 and thus forms the cathode. Band 18 is driven by worms 46 and 47 which mesh with worm gears 48 and 49 secured to the vertical rollers 23 and 24 respectively. Rollers 20 and 25 are driven through suitable transmission means (not shown) in synchronism with the rollers 23 and 24 to prevent the band 18 from binding during the transitions from the horizontal lay to the vertical and vice versa. The band is driven sufficiently rapidly to prevent the nitric-acid bath 33 from eroding the exposed electrodeposition surfaces thereof.

In operation, silver metal goes into solution at the anodes 22 while silver oxide is deposited upon the flexible cathod 18 therebetween. After compression by rollers 24 and 25, the silver-oxide mass is washed in the water bath 27 and converted to silver nitrate in compartment 16, while the band, after a subsequent washing, is returned in a continuous process to the alkaline electrolyte 21.

Example A stainless-steel band 18, 1 meter in width, is advanced through the compartment 14 at a speed of 3 meters per minute. Silver oxide is deposited upon the band with a current density of 0.45 ampere per square centimeter from a 28.8% solution of potassium hydroxide saturated with potassium acetate and having a specific gravity of approximately 1.295 at room temperature. The anodes 22 are positioned 40 centimeters from the band 18 so that a deposition voltage of only about 12 volts is re quired. After a run of about 25 minutes, kg. of silver nitrate may be removed from compartment 16.

The invention as described and illustrated is believed to admit of many modifications and variations which are within the ability of persons skilled in the art and are intended to be included with the spirit and scope of the invention, except as further limited by the appended claims.

I claim:

1. A process for producing silver nitrate, comprising the steps of depositing a loosely adherent mass of silver oxide on a conductive endless band by passing said band through an electrolyte containing the ions of a soluble silver salt while maintaining said band at a negative potential with respect to a silver anode in said electrolyte,

compressing said mass against said band, passing the band carrying said mass through a heated nitric-acid bath whereby said silver oxide is converted to silver nitrate, and stripping said silver nitrate otf said band 2. A process according to claim 1, further comprising the steps of washing said band in water before and after its passage through said nitric-acid bath.

3. Process according to claim 1 wherein said band is moved through said electrolyte in a substantially vertical position whereby said coating is deposited on both of its sides.

4. A process according to claim 1 wherein said band is made of a material which is substantially inert in said electrolyte and in said bath.

5. A process according to claim 4 wherein said ma terial is stainless steel.

6. A process according to claim 1 wherein said band is advanced continuously through said electrolyte and said bath.

References Cited by the Examiner UNITED STATES PATENTS 748,609 1/1904 Hunt 20496 1,950,096 3/1934 Yaeger 204202 2,019,994 11/1935 Rhodes 204-206 X 2,581,519 1/1952 Critekley 23102 2,940,828 6/1960 Moede l23102 3,014,790 12/ 1961 Palmquist 23260 3,057,696 10/1962 McLeod 23-260 OTHER REFERENCES Mellor: A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans, Green and Co. 1928, vol. 8, page 595.

MARURICE A. BRINDISI, Primary Examiner.

GEORGE D. MITCHELL, Examiner. 

1. A PROCESS FOR PRODUCING SILVER NITRATE, COMPRISING THE STEPS OF DEPOSITING A LOOSELY ADHERENT MASS OF SILVER OXIDE ON A CONDUCTIVE ENDLESS BAND BY PASSED SAID BAND THROUGH AN ELECTROLYTE CONTAINING THE IONS OF A SOLUBLE SILVER SALT WHILE MAINTAINING SAID BAND AT A NEGATIVE POTENTIAL WITH RESPECT TO A SILVER ANODE IN SAID ELECTROLYTE, COMPRESSING SAID MASS AGAINST SAID BAND, PASSING THE BAND CARRYING SAID MASS THROUGH A HEATED NITRIC-ACID BATH WHEREBY SAID SILVER OXIDE IS CONVERTED TO SILVER NITRATE, AND STRIPPING SAID SILVER NITRATE OFF SAID BAND. 